User guide

MDX-ETHERNET User Guide 11
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4.7 Network topology
4.7.1 Hubs
A hub provides a basic connection between network devices. Each device is connected
to one port on the hub. Any data sent by a device is then sent to all ports on the hub.
The use of hubs is not recommended for use within control systems due to the
increased possibility of collisions. Collisions can cause delays in data transmission and
are best avoided, in severe cases a single node can prevent other nodes on the same
hub (or collision domain) from accessing the network.
If using hubs or repeaters you must ensure that the path variability value and
propagation equivalent values are checked. This is, however, beyond the scope of this
document.
4.7.2 Switches
Switches offer a better solution to hubs, because after initially learning the addresses of
connected devices the switch will only send data to the port that has the addressed
device connected to it, thus reducing network traffic and possible collisions. The
difference in price between the hub and a switch means that in almost all cases the
switch is the preferred choice. Some managed switches allow the switching of data to
be controlled and monitored, this may be of particular importance on large or high
performance systems.
4.7.3 Routers
A router is used to communicate between two physical networks (or subnets) and
provides some degree of security by allowing only defined connections between the two
networks. A typical use would be connecting the office and manufacturing networks or
connecting a network to an ISP (Internet Service Provider). A router is sometimes
known as a gateway as it provides a "gateway" between two networks. It is generally
recommended that a firewall is used when connecting networks as this provides
additional security features.
4.7.4 Firewalls
A firewall allows separate networks to be connected together in a similar way to a router.
The firewall however offers significantly more security features and control. Typical
features include address translation, port filtering, protocol filtering, URL filtering, port
mapping, service attack prevention, monitoring and virus scanning. This is usually the
preferred method of allowing traffic from a manufacturing network to the business
network. The setup and installation of the firewall should be done by a suitably qualified
engineer and is beyond the scope of this document.
LEROY-SOMER do not recommend the use of un-switched hubs.
NOTE
Some switches require a certain time to initialise (typically 30 to 60 seconds) if MDX-
ETHERNET is reset.
NOTE